| The altered water regime in the Three Gorges Reservoir of the Yangtze River, China, has caused many serious environmental problems, to restore the riparian vegetation is of great significance.Among them, as research has shown, the cadmium pollution in this region is serious, so the vegetation restoration and phytoremediation of Cd contaminated soil is of great significance in this region. Research has shown that Salix variegata can tolerate both flooding and heavy metal stress.Theoretically speaking, S. variegata can therefore be a suitable species to restore the vegetation and remedy Cd contaminated soil in the Three Gorges Reservoir Region. In order to explore the phytoremediation of cadmium contaminated soil by S. variegate under flooding, the soil chemical property, the speciation of Cd in the soil, the quantity of microorganisms and the enzymatic activities of the soils were researched. Factorial experimental treatments applied included two water regimes and four cadmium concentrations. The water treatments included ambient water supply(CK) and light flooding(FL). The cadmium additions included control(0 mg·kg-1), low concentration(0.5mg·kg-1, middle concentration(2 mg·kg-1) and high concentration(10 mg·kg-1), and each treatment involved a group with planted S. variegata shoots and a reference group without plants. The results of this research showed that:1. Effect of S. variegata to the chemical characeristics of Cd contaminated soils under flooding.The pH value of the soils of both 30 th day and 60 th day in S. variegata was significantly lower than that of non-plant groups(P<0.01). The pH value of the flooding group was lower than that of the normal water supply treatment on the 60 th day treatment(P<0.01). Cd treatment had significantly affect the pH value on the 30 th day treatment(P<0.01). Planting S. variegata had no significant effect on organic matter(P>0.05). Water treatment had significant affect the organic matter on the60 th day treatment(P<0.05). The contents of total N on 30 th day, the contents of total N and total K of the 60 th day treatment of S. variegata group were significantly lower than that of non-plant group(P<0.05). The contents of total P on 30 th day and total N, total P and total K of soils on the 60 th dayof treatment of the flooding group were significantly lower than that of the normal water supply treatment(P<0.05). It can be concluded from the results: the nutrients of the soils could be dissolved out by flooding. Thereby, the risk of eutrophication of the water could be increased by flooding. The absorption ability of nutrient of S. variegata is pretty good, so, the planting of S.variegata was able to lower the risk of eutrophication of the water.The contents of availiable N on 60 th day treatment were much higher than that of non-plant groups(P<0.05); The content of availiable N on both of the 30 th and 60 th day treatment were much higher than that of normal water supply groups(P<0.05). The content of availiable P on 30 th day treatment were much lower than that of normal water supply groups(P<0.05). The contents of availiable K of both of the 30 th and 60 th day treatment were significantly affected by planting S. variegata and flooding. The Cd treatment had little significantly effect on the total contents and availiable contents of nutrients on the soils(P>0.05). The Cd treatment had little effect on the chemical properties of the S. variegata planting soils, so, from the perspective of the chemical properties in the soils, it can be concluded that S. variegata can adapt the stress of Cd in the soils.2. Effect of Cd concentration and Cd speciation of rhizosphere soils of S. variegata under flooding.With the increasing Cd concentration in the soil, the labile fractions of Cd increased significantly,meanwhile the Cd bioavailability increased significantly, and the environmental risk could be increased significantly. Water treatment significantly affected the speciation of Cd in the soil, and the mobility factors(MF values) under the flooding treatment were much less than those under the ambient water treatment at days 30 and 60 of the experiment(P<0.05). Furthermore, the phytoremediation of Cd-contaminated soil under environmental flooding was very different from that of the soil under ambient water supply. S. variegata did not affect the total Cd concentration or Cd speciation of the soil samples taken on day 30 of the experiment, whereas it significantly reduced the total Cd concentration of the soil day 60 under the FL treatment. The exchangeable and carbonate-bound Cd concentrations of soil from both the CK and FL treatments on day 60 were also significantly affected by S. variegata. The MF value of the soil samples was significantly reduced by S. variegata compared with the non-planted groups, and the average decline was 2.0%, 4.12% in treatments of PCK and PFL on day 30 of the experiment, and 9.71%, 9.32% in treatments of PCK and PFL on day 60 of the experiment, respectively. There are two primary reasons explaining why the total Cd concentration of the soil samples was not considerably affected by S. variegata shoots:(1) The duration of the incubation experiment was too short;(2) the biomass of the S. variegata shoots was relatively small. However, S. variegata significantly reduced the MF value both in the ambient water and flooding treatment, but did not differ significantly between both treatments. Inconclusion, S. variegata could be a suitable species for the phytoremediation of Cd-contaminated areas under environmental flooding.3. Effect of S. variegata to the quantity of microorganisms of Cd contaminated soils under flooding.The quantity of bacteria in the soils on the 60 th day of the experiment was significantly increased by S. Variegata treatment(P<0.05). The quantity of bacteria and fungi on the 60 th day, quantity of actinomycetes and fungi of the soils on the 120 th day of the experiment of the flooding groups were significantly lower than that of normal water supply groups(P<0.05). The Cd treatment had little effect on the quantity of microorganisms(P>0.05). The anoxic environment could explain why the quantity of microorganisms of flooding groups was much lower than that of the normal water supply groups. There are two primary reasons to explain why the quantity of microorganisms was little affected by Cd treatment:(1) The concentration of the Cd treatment was not high enough.(2) The ability of tolerance to the Cd stress of this soils was too strong. The effect on the quantity of microorganisms by planting S. variegata was as not as good as by planting hyperaccumulator, the concentration of Cd treatment had no negative effect on the quantity of microorganisms could be an important reason.4. Effect of S. variegata to the soil enzymatic activities of Cd contaminated soils under flooding.The experimental results of the enzyme activities of this research are as follows: Phosphatase activity in the soils on the 60 th day and urease activity, invertase activity in the soils on the 120 th day were significantly improved by planting S. variegata(P<0.05). The phosphatase activity of the soils on the 60 th day and the urease activity, invertase activity, phosphatase activity of the soils on the120 th day were significantly reduced by flooding. The phosphatase activity of the soils on the 60 th day and the urease activity of the soils on the 120 th day were significantly affected by Cd treatment.The enzymatic activities in soils was raised in the low Cd concentration treatment, and then reduced with the increased of the Cd concentration. Planting of S. variegata could improve the enzymatic activities in soils of both the normal water supply groups and flooding groups. The correlations between quantity of microorganisms and enzymatic activities of soils are quite well, and both of these two indexes were sensitive to Cd stress, so, it is much more reasonable to combine these two indexes to estimate the level of Cd pollution and effect of phytoremediation.From all the results stated above, we can conclude that:(1) The survival rate of S. variegata was100% during our experiment.(2) The nutrients of the soils could be dissolved out by flooding treatment, the risk of eutrophication of the water could be increased, but the absorption ability of nutrient of S. variegata is pretty good, so, planting of S. variegata could lower the risk of eutrophication of the water.(3) The absorption ability of Cd in the soils of S. variegata under theflooding environment was as good as the absorption ability of the normal water supply treatment, so,S. variegata could be a suitable species for phytoremediation of Cd contaminated areas under environmental flooding.(4) The effect on the quantity of microorganisms by planting of S. variegata was as not as good as by planting hyperaccumulator, the concentration of Cd treatment treatment had not negative effect on the quantity of microorganisms could be an important reason.(5) The enzymatic activities in soils were reduced by flooding and Cd treatment, while, planting of S.variegata could improve the enzymatic activities in soils. All in all, S. variegata has a bright application prospect on Cd pollution soils under flooding environment. |
PDF Full Text Request